The United States Geological Survey (USGS) just released in May 2019 an update to the National Land Cover Database (NLCD), which details land cover change for the lower 48 states. (Related: USGS Releases Two Decades of Land Cover Change for the Contiguous United States) The results show significant change to the lower 48 since 2001.[1]
The key overall results shows that 7.6% of land in the United States has changed its land cover category at least once since 2001. Most of the change have affected forest cover, with reforesting and harvesting in the Southeast, while fire, pests, and forest harvest have at times devastated parts of the West.
The maps cover 2001, 2004, 2006, 2008, 2011, 2013 and 2016. Tree canopy is now measured along with urban imperviousness, which affects drainage and watersheds. Unlike previous maps, the current updates indicate shrub, bare areas, and grasslands in the West.
There was loss of shrub lands in the West, while the northern prairies have witnessed increased farmlands. Overall, throughout the United States, urban expansion is evident, particularly suburbanization and widening metropolitan areas.
The data are based on Landsat images taken, covering a spatial resolution of 30-meters. The changes can be viewed in a 1 x 1 km grid across the entire lower 48 states. The NLCD is a GIS dataset that is seen as critical for monitoring the impact of fires, particularly increased wildfires in the West, general health of ecosystems, biodiversity monitoring, and increasing effects of climate change on land cover.
The areas of Alaska, Hawaii and Puerto Rico have not been updated, but will be updated sometime in the near future.
Other sets of key changes in the 2016 database include moving from simple assessment of land cover to monitoring and trend assessment. Landsat imagery from every five years are covered and many comparable patterns noticed in the 2011 assessment are also evident in the 2016 assessment and updated NLCD.
Since 2011, two pairs of Landsat imagery have been used, which enhances the accuracy of the composite spectral image. Enhanced training sets have also been used to improve automated classification; this includes using the NASS CDL training data.[2]
For the 2016 data, training data were tested using the World Reference System-2 path/row data set, which showed agreement between 71-97% agreement for land cover classification. Data intervals are also now updated every 2-3 years, to improve the temporal resolution of the aggregate data.[3]
This would be particularly useful to better capture trends such as wildfires, as these tend to be more rapid events that create more abrupt land cover change, while more direct, anthropogenic land use change has tended to occur over longer intervals.
Furthermore, Landsat 7 will now be integrated into the data set, with Landsat 5 having been deactivated in 2013. With the changes in 2011, and recent changes for the 2016 data, including better training data for 2016, overall classification should now be improved from previous datasets. The NLCD 2016 data can be accessed from the Multi-Resolution Land Cover Characteristics (MRLC) Consortium website.[4]
It might be too early to say how the NLCD 2016 will be used. However, key changes in the West and Southeast, with land use and land cover change evident in relation to forest harvesting, forest growth, the impact of wildfires, increased farmland, and urbanization being the most evident changes from 2001 to 2016.
Improved automated classification, with better and more complete training sets, means that we can also more accurately map land cover change across the United States, improving monitoring and possibly forecasting of future land cover change based on existing land use and climate trends. The next map update will cover 2021, with current trends suggesting climate change, agriculture, urbanization, forest change, and wildfires continuing to show major effects across the United States.
References
[1] For more on the latest USGS updates, see: https://www.usgs.gov/news/new-land-cover-maps-depict-15-years-change-across-america
[2] For more on the methods used in 2011, see: Homer, C., J. Dewitz, L. Yang, S. Jin, P. Danielson, G. Xian, J. Coulston, N. Herold, J. Wickham, and K. Megown. (2015). Completion of the 2011 National Land Cover Database for the Conterminous United States – Representing a Decade of Land Cover Change Information. PHOTOGRAMMETRIC ENGINEERING AND REMOTE SENSING. American Society for Photogrammetry and Remote Sensing, Bethesda, MD, 81(0):345-354.
[3] For more training data used for the 2016 results, see: Yang, L., Jin, S., Danielson, P., Homer, C., Gass, L., Bender, S.M., Case, A., Costello, C., Dewitz, J., Fry, J., Funk, M., Granneman, B., Liknes, G.C., Rigge, M., Xian, G., 2018. A new generation of the United States National Land Cover Database: Requirements, research priorities, design, and implementation strategies. ISPRS Journal of Photogrammetry and Remote Sensing 146, 108–123. https://doi.org/10.1016/j.isprsjprs.2018.09.006.
[4] The MRLC is available here: https://www.mrlc.gov/
